1. Field of the Invention
The present invention relates to a gas adsorption sheet having a deodorizing function and an air-purifying filter having a dust-removing function and a deodorizing function, and relates to a method for producing the gas adsorption sheet.
2. Description of the Related Art
Heretofore, a granular activated carbon has generally been used for purification or deodorizing of a gas or liquid because of its cheap price, good air permeability and strong adsorbability for a solute in an aerosol or solution.
As one of techniques of applying such a granular activated carbon to an air-purifying filter, there is a sheet-like adsorption material to be used by passing an air to be purified through the sheet in a direction perpendicular to the sheet surface. However, it is not easy to form into a filter by using the granular activated carbon alone in such a sheet-like adsorption material. Even if the filter could be formed by using some carrier, there arises a problem that the granular activated carbon is easily released and lost from the carrier. Under these circumstances, it has been required to develop a technique capable of effectively using the above granular activated carbon as a raw material of the air-purifying filter.
On the other hand, with the increase of a problem about dust contamination, there has been required an air-purifying filter comprising a filter having a dust-removing function and a filter having a deodorizing function in combination. However, in the configuration wherein a dust-removing filter and a deodorizing filter are separately provided, there was a problem about the space or a problem that the cost is liable to become higher.
As a technique of effectively using the granular activated carbon as a raw material of the air-purifying filter, there is known a method of carrying an activated carbon with a non-woven fabric, a net-like substrate or polyurethane foam at present.
For example, JP-A-48-72088 discloses a gas adsorption sheet obtained by applying an adhesive on a non-woven sheet and distributing an activated carbon thereon. However, in the case of such a gas adsorption sheet, when the activated carbon strongly fixed is required, the pore surface of the non-woven fabric must be coated with a large amount of the adhesive. As a result, the air permeation resistance becomes higher and there was a problem that early clogging is liable to be caused by comparatively coarse powders such as pollen, sand dust, spike dust and the like.
JP-A-9-285531 discloses a gas adsorption sheet obtained by coating a binder on a net-like substrate to reduce pressure loss and then carrying an activated carbon, while JP-B-4-35201 discloses a gas adsorption sheet obtained by coating a binder on a polyurethane foam and then carrying an activated carbon. However, in order to put such a gas adsorption sheet to practical use as a filter, it is necessary to further laminate some sheet for preventing an activated carbon from falling off at least on the downstream side. Since the sheet for preventing an activated carbon from falling off is peeled off by a wind pressure during use even if the sheet is merely laminated without fixing, it is necessary to fix it by some method. One method includes a method using an adhesive. In this method, however, the same phenomenon as that described above occurs and the air permeation resistance becomes higher. A method using no binder includes an entangling method using a needle punch. However, since a fixing force of the activated carbon is small, the falling off degree of the activated carbon becomes larger to lower the deodorizing capability. Therefore, it was a problem on practical use.
Also, the specification of U.S. Pat. No. 5,605,746 discloses a fibrous structure comprising a non-woven fiber matrix formed from composite fibers of a heat adhesive fiber and the other fiber, and a granular activated carbon or the like entrapped in and fusion bonded to the matrix. In addition, the specification of U.S. Pat. No. 5,674,339 discloses a production method of such a fibrous structure. The production method comprises steps of dry forming an open fibrous web structure comprising the heat adhesive fiber and the other fiber, distributing and entrapping the granular activated carbon having a suitable size, weight, and the like in the web structure, and heating to fix the granular activated carbon or the like to the web structure by melt bonding.
However, in such a method for fixing the granular activated carbon by heat adhesive fiber, it is difficult to fix it with reliability so as not to fall off. Therefore strongly fixing of the granular activated carbon and adsorption or air-permeation property can not easily coexist as well as the above prior arts.
As explained above, a satisfactory gas adsorption sheet or air-purifying filter which firmly carries a granular activated carbon and is superior in adsorbability to a gas with bad odor and folding processability, and which causes small pressure loss and little clogging with dust, is not still obtained at present.
On the other hand, various filters obtained by integrally laminating a dust-removing filter and a deodorizing filter have been proposed to solve the problem about insufficient dust-removing performance.
For example, JP-A-4-74505 discloses a filter obtained by bonding a dust-removing filter using an electret non-woven fabric and a deodorizing filter formed by using a suspension of a granular activated carbon in a latex as an adhesive, and integrally forming the resultant into a shape of pleats. However, the electret non-woven fabric disclosed in this publication is a so-called span-bond type electret non-woven fabric. In the case of this non-woven fabric, the dust-removing function is insufficient. Since early clogging occurs to cause rapid increase of pressure loss, a life time of the filter is over before that of the deodorizing sheet is over. Therefore, it was not satisfactory in view of the filter function and economy.
JP-A-7-265640 discloses a filter wherein dust is three-dimensionally trapped by raising an electret fiber, thereby making it hard to cause clogging. The electret fiber disclosed in this publication is also a so-called span-bond type electret filter and, therefore, the charge rate was low and satisfactory dust-removing efficiency could not be obtained.
As described above, there is still to be obtained a filter having a combined function of dust-removing and deodorizing, which exerts a sufficient dust-removing effect and has a long filter life, at present.
Under these circumstances, the present invention has been accomplished. That is, an object of the present invention is to provide a gas adsorption sheet having good air permeability and excellent retention of an activated carbon, and an air-purifying filter whose dust-removing performance can be maintained for a long period to provide a method for producing the gas adsorption sheet.
To attain the above object, the present inventors have studied intensively about a structure for retaining the granular activated carbon by using a fiber and formation thereof. As a result, they have found that a gas adsorption sheet having good fixing strength of the granular activated carbon and good air permeability can be obtained by substantially fixing the granular activated carbon using a supporting fiber, not fixing the activated carbon using an adhesive.
The present inventors have also found that the above sheet can be preferably produced by mixing the granular activated carbon with the supporting fiber and subjecting the mixture to a wet paper method. In that case, they have found that it becomes possible to make the structure for retaining the granular activated carbon using the supporting fiber more strong and stable by adding a small amount of an adhesive fiber, in addition to the above granular activated carbon and supporting fiber.
The present invention provides a gas adsorption sheet characterized in that the deodorizing sheet and electret non-woven fabric are integrally laminated and, furthermore, the electret fiber is a film split type electret fiber. The present invention also provides an air-purifying filter characterized in that the above gas adsorption sheet is formed in a shape of pleats or wave.
The present inventors have further studied on the basis of the above knowledge, thus completing the present invention.
The above object can be attained by the present invention as described below.
The gas adsorption sheet of the present invention is a gas adsorption sheet having a granular activated carbon-containing sheet, said granular activated carbon-containing sheet comprising a granular activated carbon having an average particle diameter of 60 to 600 xcexcm, a supporting fiber for fixing the granular activated carbon in contact with it, and an adhesive fiber which mainly contributes to shape retention. A gas adsorption sheet having good fixing strength of the granular activated carbon and good air permeability can be obtained by substantially fixing the granular activated carbon using a supporting fiber, not fixing the activated carbon using an adhesive. Thereby it becomes possible to minimize the amount of the adhesive component such as adhesive and to get the intrinsic deodorizing performance of the granular activated carbon to the utmost degree and, at the same time, it becomes possible to maintain the air permeation resistance of the whole sheet.
In the above sheet, the granular activated carbon-containing sheet preferably comprises a surface layer and a back layer formed on the back of the surface layer, the surface layer containing less granular activated carbon than the back layer. Such a surface layer makes it possible to fold in a state of being hardly contacted with the granular activated carbon. Also it becomes possible to surely prevent the granular activated carbon from flowing out by providing the surface layer on the downstream side on air permeation. From such a point of view, the sheet has a surface layer containing no granular activated carbon, more preferably.
It is preferred that the granular activated carbon-containing sheet is integrally formed by wet bonding using a water-swelling fiber as the adhesive fiber. In the case of such wet bonding, since the attaching force between the supporting fiber and granular activated carbon becomes larger on drying, the granular activated carbon is fixed in the sheet, integrally and firmly, it becomes very hard to arise falling off of the granular activated carbon upon folding.
With respect to the supporting fiber, an outer surface area is preferably not more than 1 m2/g, a fiber length is from 3 to 20 mm and a density is from 0.8 to 1.7 g/cc. Thereby the air permeability, adsorption performance and paper-making property of the resulting gas adsorption sheet are improved.
The granular activated carbon-containing sheet preferably contains the granular activated carbon in an amount of 30 to 80% by weight based on the total weight. Thereby the adsorption performance and strength of the resulting gas adsorption sheet are improved.
The granular activated carbon-containing sheet is preferably provided with small pores capable of substantially permeating an air in a thickness direction. By forming these small pores, it is possible to further reduce pressure loss and to largely enhance the dust retention amount. Furthermore, it is possible to largely enhance the life of the gas adsorption sheet.
In that case, an average open area per one pore of the small pores is preferably from 0.5 to 3 mm2. The number of the small pores is preferably from 1 to 20 per 1 cm2 of the granular activated carbon-containing sheet. Furthermore, a porosity of the small pores is preferably from 3 to 10%.
In the present invention, it is preferred to further comprise an air-permeable sheet, in addition to the granular activated carbon-containing sheet described above. Because it is possible to further secure the prevention of the granular activated carbon from falling off upon folding or use as the filter. Considering a packing density in the sheet thickness direction, a coarse or dense state can be realized. Therefore, since dust can be retained comparative uniformly in the sheet thickness direction, when the dust is introduced from the upstream side, it becomes hard to increase pressure loss due to early clogging. Accordingly, it is preferred that the air-permeable sheet is laminated on the back layer of the granular activated carbon-containing sheet.
In that case, the air-permeable sheet preferably comprises a non-woven fabric made of a film split type electret fiber as a main component. Thereby it is possible to enhance the removing effect to sub-micron particles.
It is preferred that the air-permeable sheet is further provided with a cover sheet in the form of a non-woven fabric, woven fabric or net. Thereby handling properties as the filter, such as stabilization of the dust-removing efficiency, stabilization of dust retention amount, prevention of fuzz of film split type electret fiber, etc. can be further improved.
A packing density of the air-permeable sheet is preferably from 0.01 to 0.20 g/cc. Thereby the air permeability, dust retention amount, strength and handling properties are further improved.
On the other hand, the air-purifying filter of the present invention is an air-purifying filter obtained by forming a gas adsorption sheet into a shape of pleats or wave. This filter has good air permeability and excellent dust-removing performance and can maintain excellent dust-removing performance for a long period because the gas adsorption sheet of the present invention is used. Furthermore, an effective filtration area can be enhanced by forming into a shape of pleats or wave without falling off the granular activated carbon on processing as described above.
In that case, thickness of the filter formed is preferably from 10 to 400 mm and a distance between crests is preferably from 2 to 30 mm.
On the other hand, the production method of the present invention is a method for producing a gas adsorption sheet, which comprises a step of forming a granular activated carbon-containing sheet, said step comprising the steps of preparing an aqueous slurry containing a granular activated carbon having an average particle diameter of 60 to 600 xcexcm, a supporting fiber and a water-swelling adhesive fiber, spreading the aqueous slurry in a sheet-like form, and mechanically dewatering and drying the aqueous slurry spread.
According to this production method, since an amount of the adhesive component required is very small, a granular activated carbon-containing sheet having low air permeation resistance can be formed and characteristics of the granular activated carbon can be utilized to the utmost degree. By utilizing a difference in sedimentation ratio of the solid content, it is also possible to form a surface layer containing less granular activated carbon than the back layer.